I no longer have elbow pain during endurance races w/ the wider bars...my theory involves less strain on the triceps since the hands are further apart. Think of the difference in feel between a narrow grip pushup and wide grip. This is on a ss, I think I would cut off an inch or two on a geared bike.

I was on a 580mm flat bar but recently switched to a 635mm low rise and won't be going back. Descending is much better on the wider low rise bar and haven't noticed any loss climbing or riding singletrack. Did take a couple of weeks to get used to them.

I found (for me and my situation) the 640mm is the best, I have 660 and it is a little wide, and i have hit some trees. my new bike has 600 and it hurts my wrists when out of the saddle. You also need to consider stem lenght. for me 90mm for 640-660 and for the 600 it is a 105 stem. this leaves the same distance between the nose of the seat and middle of the grips on the bars.